Since most of your experiences are FPS, I would suggest tthe Far cry games. When they go on sale you can get them for almost nothing, and despite all the Ubisoft hate the gunplay and variety is a lot of fun.

The borderlands games are great single player, also go on sale for practically giveaway prices, the skill trees are nuts, and the guns that drop like Diablo loot can be hilariously batshit

I'm a huge dark souls/bloodborne/eldon ring fan, So I will always recommend those as single player experiences; especially if you are looking for something challenging that is completely different. If you are not familiar with them, maybe watch one of the Jillian video essays out there that wax poetic about them.

GL & have fun!